1. Field of the Invention
In order to detect the spatial direction of an incoming electromagnetic energy signal wave, interferometers are required. In particular, interferometers are used in radio astronomy for detecting celestial bodies or in range instrumentation for tracking moving objects such as aircraft, spacecraft, and missiles. One type of interferometer is the long-baseline type which permits a precise determination of the signal's direction of arrival. However, long-baseline interferometers are generally susceptible to a phenomenon which is generally referred to as ambiguities. The elimination of the ambiguities is the subject of the present invention.
2. Description of the Prior Art
In general, one-dimensional, long-baseline interferometers are common in the prior art. For example, such devices are disclosed in U.S. Pat. No. 4,213,131, by Kaiser, Jr., U.S. Pat. No. 4,136,342, by Alcock et al., and U.S. Pat. No. 3,852,754, by Worrell. More particularly, two-dimensional, long-baseline inteferometers are also disclosed in the prior art. For example, such devices are shown and described in E. Ehling, "Range Instrumentation", Prentice-Hall, Inc., Englewood Cliffs, N.J., 1967, pages 291-347. In addition, a two-dimensional interferometer is disclosed and described in U.S. Pat. No. 3,935,575, by Leisterer et al., and as an alternative embodiment in U.S. Pat. No. 4,213,131, by Kaiser, Jr. However, the two-dimensional interferometers in the prior art are deficient in several aspects.
One deficiency is that the prior art devices require a multiplicity of antenna elements, generally at least five or more, in order to function as a two-dimensional interferometer. For the two-dimensional interferometers disclosed in E. Ehling, "Range Instrumentation", supra, seven to ten antenna elements are used in each interferometer.
Another deficiency is that the prior art devices are incapable of eliminating ambiguities when detecting the incoming radio-frequency waves. For example, in the two-dimensional interferometer disclosed in U.S. Pat. No. 3,935,575, by Leisterer et al., elimination of ambiguities is limited to a short-baseline interferometer.
A further deficiency is that the prior art devices are dependent on the wavelength of the incoming radio-frequency waves. In the interferometers disclosed in U.S. Pat. No. 4,213,131, by Kaiser, Jr. and U.S. Pat. No. 3,852,754, by Worrell, the antenna elements are spaced apart by integral multiples of one-quarter and one-half wavelengths, respectively.